This application bases priority on German application no. 100 51 715.3-52 filed on Oct. 12, 2000.
1. Field of the Invention
The invention relates to a procedure for the systematic determination of substances with particle character, moving with a vertical component in a flowing fluid, for the mass accumulation of particle groups in a calibrated sediment trap exposed to flow with knowledge of the actual values of the trap and flow parameters, and to an apparatus for executing this procedure.
2. Description of Prior Art
Materials with particle character, moving with a vertical component in flowing waters and oceans, play an essential role in sedimentological, biogeochemical, biological, climate-relevant, morphological and many other processes. Their systematic determination leads, among others, to detailed results regarding the global cycle of organic carbon and nutrients. Likewise, many technical installations with moving fluids require information about the vertical mass flux of embedded substances with particle character. In connection with the present text, the term xe2x80x9cmaterials with particle characterxe2x80x9d is to be understood as representing both particles consisting of solids (either fully solid or aggregated) in the sense of xe2x80x9cparticlesxe2x80x9d as well as particles consisting of gas bubbles or solid-gas combinations. Depending on relative density of the particle type it experiences, either a sinking force (sinking materials) or a buoyancy force (raising materials). In the state-of-the-art, the major group considered in connection with sediment traps is that of sinking particles, which is the reason why the following text mostly refers to this type. The devices used according to the state-of-the-art for determination of the vertical mass flux (so called xe2x80x9csediment trapsxe2x80x9d) collectxe2x80x94even at the same measuring locationxe2x80x94individually different amounts of sinking particles due to the different collection and extraction procedures and apparatus used. For these cases the prominent task was to determine qualitatively and quantitatively the composition of the pool of a sinking substance with particle character rather than to determine the xe2x80x9cin-situ mass fluxxe2x80x9d of the different sinking particle groups it is comprised of in the sense of knowledge of the exact values of actually sinking mass and the detailed sinking processes in the fluid. The equation used accordingly by which the so called xe2x80x9ctrap fluxxe2x80x9d is determined by dividing the particle mass collected in the sediment trap by collection time and total area of the trap aperture (the opening area of a sediment trap), only permits one to calculate the mass flux of sinking particles and related variables in the open water column for fluids without motion.
In case the fluid carrying particles moves with a horizontal flow component towards the trap aperture which is typically horizontally aligned, then the actual collection process in the trap is strongly altered by the approach velocity in comparison with the sinking or sedimentation process in the free water column or at the bottom, respectively. A circulation pattern of the fluid is generated which is coupled to the approaching flow in the entrance zone of the trap, which thus is continuously flushed by new fluid. This xe2x80x9ccirculation zonexe2x80x9d permits particles embedded in the fluid to reach not only gravimetrically but also advectively the inner space of the trap from where a fraction settles. This holds for particles with a sinking speed larger than 5 m/day, slower-sinking particles, whose fraction of the total mass flux is mostly less than 10%, can only be collected by other techniques. The flow-influenced collection process of sinking substances with particle character cannot be quantified by the classical trap flux equation for non-moving fluid (mentioned above) and is reflected mathematically in semi-empirical xe2x80x9caccumulation equationsxe2x80x9d, by the help of which from the measured mass accumulation of a known, calibrated type of trap the in-situ mass flux of a single particle group can be calculated.
Here, the term xe2x80x9ctype of trapxe2x80x9d is to be understood as a trap parameter specification based on coefficients related to an individual trap as sub-group of a xe2x80x9ctrap familyxe2x80x9d as the higher classification level. Such accumulation equations exist for different trap families, in particular for cylindrical and conical traps (see Reference 1 of Gust et al: xe2x80x9cMooring Line Motions and Sediment Trap Hydromechanics: In-Situ Intercomparison of Three Common Deployment Designsxe2x80x9d 1994, Deep Sea Research 41, 831-857 and Reference 2 of G. Gust et al.: xe2x80x9cParticle Accumulation in a Cylindrical Trap under Laminar and Turbulent Steady Flow: An Experimental Approachxe2x80x9d 1996, Aquatic Sciences 58, 297-326) and plate traps (see Reference 3 by B. Westrich xe2x80x98Fluvialer Feststofftransport-Auswirkungen auf die Morphologie und Bedeutung fuer die Wasserguetexe2x80x99 1988, Schriftenreihe Wasser-Abwasser 22, R. Oldenbourg Verlag, Muenchen, Wien, pages 24-29). The development of additional accumulation equations for other trap families is presently a research task. These can be incorporated without problem into the invention described here since they will be based on the same basic concepts. Particularly, Reference 2 provides basic explanations for the mass accumulation of particle groups in a calibrated sediment trap under flow as basis for the invention presented here. An excerpt for the necessary understanding of the invention is offered in the following. The accumulation equations presented in References 1 and 2 show that sediment traps of cylindrical and conical geometry operate on a basically different particle-collection principle compared to plate traps with flat, hydraulically smooth or rough surfaces. For example, for plate traps (see patent DE 197 37 448 A1) the particles settle immediately from the moving fluid onto the trap surface as soon as the deposition stress is larger than the counteracting flow-induced bottom stress at the collection location. In case the flow-generated bottom stress exceeds this value, the particles cannot settle any more. If the bottom stress reaches with increasing flow speed the critical bottom stress, then the particles which had already settled on the collection surface are eroded again. The plate traps thus provide a mean value of mass for experiment duration and collection area, which depends on the deposition stress of the sinking particles, on the flow-generated bottom stress, on the critical bottom stress for erosion of deposited particles as well as on the respective initial concentration of particle groups with different settling velocities. In contrast, for sediment traps with hollow geometry the accumulation process under flow leads via a trap-internal circulation pattern which is coupled to the outer flow and whose fluid flushing rate depends on the velocity of the approaching fluid. From the deepest section of this circulation zone where the particles are carried along with the fluid while sinking, a fraction of particles as described by the so called xe2x80x9cyields functionxe2x80x9d is transferred into the underlying quiescent fluid zone where the particles are moving downwards exclusively by their sinking velocity and eventually settle on the collection surface. This type of collection process leads via the experiment duration to the cumulative increase of mass collected (mass accumulation). The quiescent zone beneath the circulation zone is required in both design and function and exists only for a proper length-width ratio of the trap geometry.
For a single, vertically aligned sediment trap the following accumulation equation holds for a substance comprised of a mixture of sinking particles (Gust et al., 1996)
MTP=xcexa3(MT)n=xcexa3y(ua,wsn)c0nt(Q(ua)+Aswsn)xe2x80x83xe2x80x83[1]
For a single conical trap, a dependency of simular mathematical structure but with additional controlling parameters is assumed:
MTP=xcexa3(MT)n=xcexa3(y(ua,wsn,TKE)c0nt(Q(ua)+AsWsn))xe2x80x83xe2x80x83[2]
For a single plate trap the following accumulation equation can be cited according to Westrich (1988):
MTP=xcexa3(MT)n=xcexa3(c0nwsn(1xe2x88x92xcfx84/xcfx84d))xe2x80x83xe2x80x83[3]
The terms used in equations [1],[2] and [3] mean:
active trap aperture As: that section of the total trap aperture A through which the particles are transported into the interior of the trap;
approach velocity ua: velocity of the approaching fluid flow in which the particles are embedded;
fluid flushing rate Q (ua): the amount of fluid transported per unit time through the trap interior, measured at the trap aperture (depends on the approach velocity and on the trap geometry);
mass accumulation MTP: total particle mass which remains in the trap during the selected time interval t under given flow, particle and environmental conditions; the particle group mass accumulation (MT)n identifies the mass of the particle group n which shows mean sinking or raising speed wsn (either as single particle or as mean value of a particle group with the running index n); the sum of all sinking/raising velocities forms the sinking/raising particle velocity spectrum with information on how the total mass accumulation MTP is distributed over the n particle groups (xe2x80x9cparticle spectrumxe2x80x9d);
particle concentration c0n: initially present mass per fluid unit volume (identified by number, volume, and density of the particles) for a particle group n;
TKE: turbulent kinetic energy, here particularly referring to the region immediately above the collection cup of conical traps;
yield function y (ua, ws) experimentally determined function describing the fraction of a particle group n which is moving with the circulation cell (describes the dynamic transport process in the circulation zone) through the trap and is retained;
xcfx84: bottom stress xcfx84 in a trap
xcfx84d: deposition stress in the trap
A relevant parameter to be derived via the mass accumulation of the systematically quantified substance with particle character in a moving fluid is the particle group in-situ mass flux (cows)n of those n particle groups which are retained in a trap and which add up to a total in-situ mass flux xcexa3(cows)n. Its determination by equation [1] with a single trap is only possible when particles of a single particle group with a single settling/raising velocity ws1 are exclusively collected. In nature, generally more than one particle group exist simultaneously, which are classified by the particle sinking/raising velocity spectrum. In case this exists, determination of the in-situ mass flux from n separate particle groups and of the in-situ total mass flux of the particle spectrum under flow by use of a single trap type is not possible. Use of such single-trap designs is presently the basis of all known devices to collect sinking particles.
Regarding the known devices for the execution of current collection and extraction procedures with existing patents, sediment traps are described which are not able to determine the in-situ vertical mass flux under flow. For example, in patent DE 29514173U1 and U.S. Pat. No. 4,869,118 an apparatus is described which is only identified for use in quiescent water. In other devices, no internal no-flow zones exist for the purely gravitational settling of particles beneath the trap-internal circulation zone. Stirred or pumped samples are investigated, instead (DE 3430263A1). Other proposed devices cannot be deployed on oceanic environments at depths in-situ (DE 9211263U1) or have insufficient geometries to prevent the resuspension of settled particles on the bottom when the approach velocity increases (DE 29520919 U1). Additionally, devices exist for the control of exclusively gravitationally operating sedimentation systems with stirrer (WO96/33000A1). The patent DE 19737448A1 describes a plate trap, for which the sedimentation equation is known and by which near the bottom a relatively unbiased collection rate of depositing material can be obtained as long as no erosion events occur during the collection time. This trap operates by another collection principle than traps with hollow geometry. Furthermore, other known patents (U.S. Pat. Nos. 4,762,009; 3,715,913) use a single trap for mass collection, in some cases also in multiples for replication of the results (DE 29520919U1). Concepts with purposefully selected different trap types are not in use.
The known state-of-the-art in collecting particles in a fluid body can be summarized such that in sediment traps with both hollow and flat geometries the mass accumulation occurs according to physical laws which are individually determined by the combination of trap type and family, flow as well as particle characteristics at the collection site. The consequently arising variation of the different controlling variables and resulting mass accumulations stretch under real-world conditions over a wide margin. To date, the use of single traps under flowxe2x80x94even at such low velocities as 1 cm/sxe2x80x94can not generate the in-situ mass flux of n particle groups comprising the substance to be determined from the mass accumulation. Even use of several traps of the same family and even type for statistical confirmation of the collection result does not lead to a conclusion about this parameter.
It is thus the task of the present invention to provide a suitable procedure to systematically determine substances of particle character in moving fluids by which the vertical particle flux and from it derivable additional particle parameters as relevant in-situ parameters can be obtained from those particle groups of the substance to be determined in the open-water column which contribute to the mass accumulation in a sediment trap. Furthermore, an apparatus is to be presented which, based on a calibrated type of sediment trap and a current meter, which is particularly suited to execute such procedure yet remains simple in design and handling. Altogether the measuring sites are not to be restricted to oceanic environments but extend to all limnic, fluvial and technical regions as well.
It is only from a thorough understanding of trap-specific mass accumulation equations of particles (or potentially also other particle-specific characteristics) that the procedure described in the invention for determination of the vertical in-situ mass flux of individual particle groups from the total mass accumulation under flow in a calibrated sediment trap can be developed. The invented procedure thus consequently represents a straight forward development of the insights which have emerged over the past years regarding the relevant in-situ parameters of substances with particle character under flow. While to date a differentiation between these relevant parameters was not possible and the in-situ determination limited to substances comprised of merely a single particle group, now all particle-type substances with an arbitrary number n of different particle groups can be identified usefully and categorically in their in-situ parameters. Relevant for the procedure is, that a number N of measuring locations is selected within the measuring region which agrees with the number n of individual particle groups which comprise the particle spectrum and where in each case the total particle spectrum contributesxe2x80x94though in different mass fractions for different trap typesxe2x80x94to the mass accumulation. Together with the sinking/raising velocity spectrum of the n different particle groups a system of equations is available via the above-cited accumulation equations with a total of N equations which can be solved for the respective values of the concentration of the n particle groups as unknown variables under knowledge of the additional calibration and measurement parameters. From this follows under combination with the associated sinking/raising speeds by calculation the respective value of the vertical in-situ mass flux of the n different particle groups as well as desired characteristic bulk parameter the total in-situ mass flux prevailing at the measurement site (for very slow-sinking particles other procedures are to be used) of the particle spectrum of the substance with particle character investigated.
An essential requirement for obtaining a solution for a system of N equations is the linear independence. An equation is not allowed to be transformable into another one merely by multiplication by a constant. This is achieved in the invented procedure by providing at the N measuring locations, the sediment traps located there are independent trap types which a clear distinction in their trap parameters, in particular their geometric dimension. Thru the geometric design, different sediment trapsxe2x80x94even under geometric similarityxe2x80x94show individual collection behavior which furthermore depends on the existing flow and particle parameters. Demonstration of geometric similarity between individual trap types has not been reported to date. Such traps would not be suited for use at the measuring locations. The selection of the trap parameters has to include adequate discrimination between the parameters such that the unavoidable measurement errors of the different parameters do not have an influence towards a linear dependence. Only a selection of individual parameters beyond this range of measuring uncertainly, which is to be established via solutions of the conditioning problem for the procedure and the apparatus, guarantees the required linear independence of the individual equations within the system of equations. All parameters used in the N equations of the linearly independent equation system have indices in accordance with the existing number n of different particle groups comprising the substance with particle character to be determined. For each of the N equations thus follows a corresponding specific set of values of the parameters involved. A variable with index is also the collection time interval, within which the different mass accumulations build up in the traps. For the collection time interval any reasonable value can be selected. For the general case it follows that the N individual traps can be operated at different times and for different duration in a regional measuring area under the condition that the other parameters are known in their time history of value assignment as well and all n different particle groups are always collected. According to a favorable realization of the invented procedure, it is provided that the start of the selected collection time intervals for each of the N particle traps is identical and all collection time intervals have the same length. In this synchronized case, it is warranted that the environmental parameters coincide for all traps. In addition, a simplification is given in the calculation of the variables when all collection times are identical.
Each of the n particle groups has a characteristic vertical velocity component (particle sinking or raising velocity) in the fluid column, also when exposed to current flow, which establishes the sinking/raising velocity spectrum. The determination of the sinking/raising velocity spectrum of the n particle groups can be established in different ways. In addition to special protocols, for example by an independent sinking velocity apparatus (settling chamber methodxe2x80x94SCM, see Reference 4 of H.-P. Kozerski 1994 xe2x80x9cPossibilities and Limitations of Sediment Traps to Measure Sedimentation and Resuspensionxe2x80x9d, Hydrobiologica 284, pp. 93-100), methods to determine the sinking/raising velocity spectrum can be chosen such as mathematical optimization techniques or procedures with access to known data sets, for example from literature sources and data banks. Particularly simple and thus advantageous is the approach, as shown in a reduction to practice of the invention, to measure the sinking/raising velocity spectrum of the n particle groups optically, particularly at a location without moving fluid. Optical procedures are simple and permit a high measuring accuracy for reproducible measurement results. The sinking/raising velocity spectrum can optically be determined at many locations within the regional measuring area to validate the single measuring values. Exist are quiescent fluid zones at the measurement location without fluid being flushed through, yet nevertheless registration of all measurable particle groups n is ensured, then in such tranquil zone optical measurements are particularly advantageous. It suffices to chose hereby only one single optical measuring site, since determination of statistical mean values is not necessary.
Already in the introduction of the description, the issue was addressed that concerning the term xe2x80x9cparticle characterxe2x80x9d of substances with particle character both sinking as well as raising substances are meant, as result of which alternatively sinking and raising velocities are introduced as well. According to another favorable realization of the invention it is thus possible, that the n different particle groups comprising the systematically to be determined substance with particle character are formed by solid particles with sinking behavior or by combinations of solid and gaseous substances of different mixing ratios with raising behavior in the moving fluid. The describing parameters are identical for sinking and raising substances, a difference is only given in the sign of the sinking and raising velocity, respectively. Thus, no different conditions arise for the invented procedure from its application for either solid or gaseous particles, it has only to be ensured that the particle types moving in directions with different vertical components are properly collected according to the applicable accumulation equations. Consequently, as a rule, either only raising or only sinking particles can be determined.
An essential element for executing the procedure is the sediment trap (or gas trap). This can belong to different families (cylinder, cone, or boundary layer resp. plate trap) and be designed differently regarding its geometric dimensions in each case. Each realization of a sediment trap thus represents an individual trap type. From the invented procedure follows immediately for the execution of the procedure an arrangement of a minimum of N individual sediment traps which are placed at least at N measuring locations within a regional measuring region with moving fluid, all of which collecting the n different, measurable particle groups. Here self-contained sediment traps can be used of the same or also of different trap family in geometric variations which are optimized for the different measuring tasks at hand and may, for example, serve also other measuring tasks or are primarily deployed in the region for this purpose. Using more traps than necessary for the collection of n particle groups merely leads to an over-defined system of equations. Then individual, most suitable traps can be selected, or the values from the excess trap can be used for error statistics, for example. In case of an under-determined equation system, the number of traps is to be augmented accordingly. Upon knowledge of the trap and flow parameters, by the invented procedure an in-situ link between these N autonomous sediment traps and the n different particle groups of the substance to be determined can be established. To proceed, the mass accumulations in the N sediment traps and the sinking velocity spectrum (or raising velocity spectrum for raising particles) of the n particle groups has to be determined. For such setting, each trap has to be fully instrumented and be handled independently.
In addition to this general type of apparatus for executing the protocol it is, however, starting from at least one calibrated sediment trap of one trap family assigned to a collection cup and a flow sensor, particularly advantageous when an invented apparatus is selected which, for the determination of the vertical in-situ flux of n different particle groups and the in-situ total mass flux of the particle spectrum, which can be derived from this, of the substance to be determined in a free water column of a regional measuring site, consists of a trap multiplett with a number N of particle traps congruent with the n particle groups with type-specific trap parameters and assigned collection cups which are oriented against the direction of the sinking/raising movement of the n particle groups. With the term xe2x80x9cmultiplettxe2x80x9d the combination of neighboring traps is expressed, all of which are used to measure the same physical quantities. Such a multiplett represents a complete measuring unit and is simple to deploy and easy to handle. Due to the close neighborhood of the individual traps, a correct positioning within a regional measuring site is ensured, such that in any case all traps collect from the same existing particle spectrum the accumulated mass. Furthermore, an individual instrumentation is not necessary. With commonly used single instruments the respective parameters can be determined for the whole multiplett. The orientation of the collection cup against the sinking or raising movement of the particles ensures that both types (either sinking or raising substances) can be collected. The respective particles to be collected move into the collection space and remain there.
Depending on the type of the existing n particle groups and the moving fluid different trap families, identified either as particle or gas traps, are particularly suited for the collection process. Thus it is a favorable realization of the apparatus to execute the invented procedure to select the N particle traps with type-specific trap parameters from different trap families. The variation by individual trap types within one trap family is necessary in order to ensure the linear independence of the coefficients of the equation system. As shown above, different trap families obey different accumulation equations, which accordingly are incorporated into a system of equations leading in same manner to a solution for the sought-after particle parameters. Additional trap and flow parameters, for example the bottom stress and the deposition stress of plate traps, are then to be determined by calibration of these traps in the laboratory or in the field, and by measurement of the flow velocity during the field investigation. On the basis of the respective trap, flow and particle parameters each trap type collects different mass accumulations according to the accompanying accumulation equation, for traps with hollow geometries (cylinder and cone traps) according to equations [1] and [2], respectively. Due to the especially advanced insights into the behavior of these trap families, which are condensed in the experimentally confirmed semi-empirical accumulation equations, it is thus according to a realization to practice particularly advantageous when at least one of the N particle traps is shaped as trap with hollow geometry, and in particular as cylinder trap with a quiescent settling zone above the collection cup. The quiescent zone, which in a hollow cylinder lies beneath both the circulation cell which can be comprised of several eddies at its deepest section for higher approach velocities and the subsequent transition zone, is a relevant space in the cylinder trap since the fluid here is free of velocity effects.
The sinking/raising velocity spectrum can be determined at varied locations in the fluid by different procedures and devices. Outside a trap it has to be considered, though, that non-trappable particle groups due to their low vertical velocity need to be compensated for. A location, at which only those particle groups are found which are also collected by a trap is the quiescent zone inside cylinder traps. A plate trap can be considered as a quiescent zone over its complete upward-facing surface. In a quiescent zone the caught particles are exposed to very little or no flow-related influences. In a further advantageous realization of the invention it is thus particularly useful when for at least one sediment trap, in particular in the quiescent zone of a particle trap with hollow geometry, an optical sensor to register the sinking/raising velocity spectrum of the n particle groups is incorporated. As optical sensor an optical recording unit (a video camera) may be selected. In this way, by simple means highly accurate, reproducible measuring results of the sinking/raising velocity spectrum can be obtained. A compensation based on measuring or mathematical techniques for non-trappable particle groups is thus not necessary.
A particular problem in the use of sediment traps is the removal of the mass accumulated from the assigned collection cups, which is also valid for the trap multiplett. In order to avoid on one hand continuous emptying which would always require a recovery of the sediment trap deployed and, on the other hand, to avoid also too large collection cups with long exposure times during which changes in the collected particle groups are unavoidable, it is of advantage according to another realization of the invention to equip the trap multiplett with a movable, particularly rotary sample magazine equipped with a multitude of sampling cups which can be assigned freely to the individual sediment traps. Such sample magazines are state-of-the-art in connection with a variety of collection devices and can be realized in a multitude of versions. Important is the unambiguous connection between the different collection cups and sets of measurement results. For the trap multiplett, linearly moving magazines with single or multiple row design can be used with appropriate parallel motion of the collection cups. For sediment trap types held in fixed positions, circular magazines can be used as well where the collection cups are mounted in a circle-shaped guide permitting slip motion on a circular path. Of advantage is in this case particularly the stable and compact arrangement of the collection cups which consequently permits a favorable spatial arrangement of the sediment traps. In the sense of unambiguous links of values collected it is furthermore useful in another favorable realization of the invention that the collection cups are opened and closed via an individual timing program which is particularly based on the occurrence of events. Such events could be reaching a particular filling level of the collection cup as well as certain environmental events such as particle clouds.
In connection with a sampling magazine and a timed control of the collection cup connections with the trap multiplett, the invented procedures for determination of substances of particle character can be automated in a particularly simple way. For example, in connection with particular environmental hazards or other harmful substances in the fluid or with particular flow events in the fluid it may additionally be useful to position the trap multiplett at different heights in the moving fluid. This can, for example, be achieved by a mooring line by which the trap multiplett can also be simply deployed and recovered. Further explanations of type and sequence of executing the procedure are to be taken from the particular description section.
Beforehand, it should be noted that the explanations presented here are based on the particle-relevant parameters, particle concentration and sinking/raising velocities. The concentration is a characteristic parameter which will always arises in connection with information about individual particle groups of the total particle spectrum. In lieu of the sinking or raising velocities the particle groups could also be classified by the behavior of so called xe2x80x9ctracer characteristicsxe2x80x9d which may be coupled to the single particles of the respective particle groups. The presentation of associated tracer fluxes would then be provided by tracer accumulation equations.